1. Field of the Invention
The present invention relates generally to rotary valve actuators and, more particularly, to apparatus and methods for a dual end stop actuator adjustment.
2. Description of the Background
Actuators, such as a piston activated pneumatic actuators, are well known in the prior art for controlling valves between an open position and a closed position. An exemplary and highly compact configuration for a pneumatic actuator is shown in U.S. Pat. No. 4,354,424, issued Oct. 19, 1982, to Sven Nordlund, which is hereby incorporated herein by reference. In that actuator, each actuator piston is provided with a rack having teeth thereon to engage corresponding teeth of an operating element. The rack includes a recess for receiving a first spring that produces a return force on the piston. The recess extends substantially into the rack of the piston. A second shorter spring is provided within a central portion of each actuator piston. Thus, one spring in each piston is offset from the center and is longer than the other spring. The offset, longer spring provides a solution to a basic problem of compact spring return actuators. Prior to this invention, such actuators did not consistently have sufficient spring force to completely close the valve.
Many valves have requirements for drift adjustments that adjust the valve position to an exactly open position and/or an exactly closed position. For these cases, if the valve is not adjusted correctly, the flow path through the valve may not be completely open or accurately closed due to an offset in the valve control element. If the flow path is not accurately controlled, then the system efficiency may be reduced, failures may occur, and maintenance costs may increase. Thus, it is often desirable to have a drift adjustment, or end stop adjustment, for adjusting the valve element for a more precise desired open and closed position. A dual end stop adjustment in the actuator, which is known in the prior art, permits adjustment of the open and closed position for the valve by adjusting the extent of movement of the valve actuator control element. For a rotational valve actuator element, it is known that a dual end stop adjustment has been used to provide two rotational stop positions for the rotational valve element.
The dual end stop adjustment may be used for many thousands of valve openings and valve closings over the lifetime of operation. Prior art end stop adjustments have a tendency change in drift or stop adjustment over time due to many openings and closings to thereby possibly cause deleterious operation of the valve system, significantly increase maintenance costs, and decrease overall system efficiency.
Prior art pneumatic actuators provide that the dual stop adjustment is sealed within the pressure zone of the actuator housing. Thus, the pressurized air, fluid, gas, and the like, used to activate the valve is present at the stop adjustment mechanism. The inventor of the present invention considers this construction to be faulty and may lead to failures and inaccuracies in the stop adjustment. One of the problems is that the pressurized housing is typically limited in size available for actuator mounting so that for a desired piston size, the thickness of the housing is also accordingly limited. The adjustment bolts must therefore extend through the relatively thin wall of the housing so as to be substantially unsupported along their length. Due to this lack of support of the bolts and the relatively thin actuator wall, there is a tendency for bending and warping in prior art actuators either in the bolts or the actuator wall. Thus, the thousands of openings and closings of the valve may well lead to an unstable or effectively non-operational drift adjustment thereby potentially causing valve and/or valve system malfunctions.
The inventor has discovered other problems with existing dual stop adjustments for pneumatic actuators. One such problem concerns end stop bolts for engaging a stop surface wherein the flat head of the end stop bolt is subject to deformation, high spots, and the like which may result in an unstable drift adjustment. Another problem discovered by the inventor relates to the mounting or adaptor plate used to secure the actuator to various types of valves which plates thereby adapt the actuator to the particular type of valve. Prior art mounting plates are supported and positioned by bolts that are subject to offsets, bending, and warping which leads to inaccuracies in the dual end stop adjustments as well as the connection to the valve which may require high accuracy to standards, such as for instance, ISO standards. Another discovered problem relates to the machining cost of drilling numerous holes in the mounting plates for support bolts. Depending on the location of the bolts, this can result in additional machining operations so that, according to the inventor, it would be desirable to achieve additional reliability and accuracy with a reduced number of mounting bolt holds. Yet another discovered problem relates to bending and offsets of the rotary drive shaft element due to torque applied to the rotary drive shaft element by the stop adjustment. Yet other discovered problems relate to stress in the mechanical supports for the dual end stop adjustment.
Consequently, there remains a need for a more reliable, consistent, and stable dual end stop adjustment that solves the above-listed unaddressed problems and other problems of prior art pneumatic actuator dual end stop adjustment mechanisms. Those skilled in the art have long sought and will appreciate the present invention which provides solutions to these and other problems.
The present invention was designed to provide more accurate and reliable operation of a pneumatic actuator to thereby more accurately control valve openings and closings over a lifetime of operation and to avoid deleterious changes that may greatly increase maintenance costs and reduce efficiency of a system of valves.
Therefore, it is an object of the present invention to provide an improved pneumatic actuator.
Another object of the present invention is to provide an improved dual end stop adjustment for a pneumatic actuator.
Yet another object of the present invention is to provide a dual end stop adjustment that does not vary in adjustment even after many, many, thousands of openings and closings of the valve.
These and other objects, features, and advantages of the present invention will become apparent from the drawings, the descriptions given herein, and the appended claims.
Therefore, the present invention may provide for a dual end stop for a pneumatic actuator wherein the pneumatic actuator comprises an actuator housing which has a zone for pressure containment. A rotary drive element is mounted for rotation within the actuator housing. The apparatus comprises elements such as a stop element mounted to the rotary drive element for rotation therewith. The stop element has a first stop surface and a second stop surface. The stop element may preferably be positioned outside of the zone for pressure containment. A first stop member is utilized for engaging the first stop surface. The first stop member, in a preferred embodiment, may be mounted outside of the zone for pressure containment. A second stop member is also utilized for engaging the second stop surface and the second stop member may also preferably be mounted outside of the zone for pressure containment.
In a preferred embodiment, a first bearing is mounted on the rotary drive element on a first side of the stop element. A second bearing is mounted on a second side of the stop member opposite to the first side. In a presently preferred embodiment, the first bearing is mounted in the actuator housing adjacent a stop assembly housing and the second bearing is mounted in the stop assembly housing.
The stop assembly housing may preferably be provided defining an aperture therein. The stop assembly housing may preferably be mounted to the actuator housing and the stop element may be positioned within the stop assembly housing. In a preferred embodiment, a first threaded portion for the first and/or second stop member engages and extends into a receptacle such that more than eighty percent of the first threaded portion extending into the receptacle may be threadably engaged so as to be supported within the receptacle.
In another aspect of the invention, a rounded, convex, or tapering end of the first and/or second stop member is engagable with the first stop surface so as to provide a defined point contact therewith.
The present invention also comprises a method for assembling/construction and may comprise steps such as providing the rotary drive element with a first stop surface and a second stop surface such that the first stop surface and the second stop surface are preferably positioned outside of the zone for pressure containment. Other steps may preferably include providing a first stop member for engaging the first stop surface to stop the rotary drive element in a first rotational position wherein the first stop member may be moveable relative to the stop surface for adjusting the first rotational position. Another step comprises providing a second stop member for engaging the second stop surface to stop the rotary drive element in a second rotational position wherein the second stop member may be moveable relative to the stop surface for adjusting the second rotational position.
Additional steps may include one or more of the following: mounting the first stop member and the second stop member within a stop assembly housing, providing an aperture within the stop assembly housing, providing a first radiuused section as part of the aperture adjacent the first stop member, providing a rounded end surface for the first stop member such that the rounded end surface is engagable with the first stop surface, and/or providing support for the first stop member such that the stop member is threadably supported at a position closely adjacent the rounded end.
In another preferred embodiment, a stop assembly housing may be sized to fit into a recess in the actuator housing. The stop assembly housing defines an aperture therein. The first stop member may be mounted within the stop assembly housing such that the first stop member is positionally adjustable and/or the second stop member may also be mounted within the stop assembly housing such that the second stop member is positionally adjustable.
The stop assembly housing is preferably machined for accuracy to a desired size to thereby mate to the recess and the recess in the actuator housing is also preferably machined to a size for receiving the stop assembly housing such that lateral movement of the stop assembly housing within the recess is prevented. In a presently preferred embodiment, the stop assembly housing and the recess are machined to a tolerance of less than or equal to onexe2x80x94one thousandth of an inch for a tight fit of the stop assembly housing within the recess.
In a preferred embodiment, an outer bearing for the rotary drive element is mounted within the stop assembly housing such that the bearing may be on an opposite side of the stop element from the actuator housing. An inner bearing may be provided for the rotary drive element mounted within the actuator housing adjacent the stop assembly housing.
A first relief groove may preferably be defined in the stop assembly housing adjacent the first convex head and/or a second relief groove may preferably be defined in the stop assembly housing adjacent the second convex head.
This aspect of the method for making and/or construction and/or assembly includes machining a receptacle in the actuator housing and machining sides of a stop assembly housing that is insertable into the receptacle. The steps of machining provide a tolerance between the receptacle and the stop assembly housing such that the stop assembly housing is prevented from lateral movement.
For relieving stress in the stop assembly housing, a method of the invention may include steps such as providing an aperture within the stop assembly housing for receiving the rotary drive, providing a first groove along the aperture, and providing a second groove along the aperture. The first stop adjustment member has a first end for engaging the first stop surface and the second stop adjustment member has a second end for engaging a second stop surface. Thus, more specifically the method may include steps such as providing the first groove adjacent the first end, and providing the second groove adjacent the second end.
The first end stop may preferably have a first tapering end which tapers to a first end point and the first tapering end surface may be engagable with the first stop surface for point contact to thereby stop the rotary drive element in a first rotational position. Likewise a second stop member may have a second tapering end which tapers to a second end point. The second tapering end surface may be engagable with the second stop surface for point contact to thereby stop the rotary drive element in a second rotational position.
The present invention preferably utilizes bearings on either side of the stop element to prevent bending of the shaft due to forces applied to the stop element. Thus, another method of the present invention may comprise steps such as installing a first bearing for the rotary drive element for supporting a first end of the rotary drive element, installing an outer bearing for the rotary drive element on an opposite side of the rotary drive element from the first end such that the outer bearing is positioned adjacent the stop element, and installing an inner bearing for the rotary drive element adjacent the stop element on an opposite side of the stop element with respect to the outer bearing.
In a preferred embodiment, the first and second stop members are supported along its length. Thus, the rotary actuator may define a first threaded elongate aperture for receiving the first elongate stop member. The first threaded aperture has a first outer opening for receiving the first elongate stop member. The first threaded elongate aperture has a length of threaded surface such that at least eighty percent of a portion of the first elongate member which extends from the first outer opening to the first end is threadably supported by the length of threaded surface. Moreover, the second elongate stop member may be threaded so as to be moveable along an axis thereof for adjustably engaging the second stop surface. The second elongate stop member has a second end for engaging the second stop surface and the rotary actuator may define a second threaded elongate aperture for receiving the second elongate stop member. The second threaded aperture has a second outer opening for receiving the second elongate stop member. The second threaded elongate aperture has a length of threaded surface such that at least eighty percent of a portion of the second elongate member which extends from the second outer opening to the second end is threadably supported by the length of threaded surface.